1. Field of the Invention
This invention relates to devices which, during fusion butt-welding between the ends of tubes, are used to provide a backing for the weld inside the tubes.
2. Description of the Prior Art
During fusion butt-welding between the ends of two tubes it is often necessary or desirable to provide a backing around the inner surfaces of the tubes adjacent and bridging their butted ends to dam the molten metal caused by the welding process. An efficient backing means can form an inner surface on the weld which requires little if any cleaning or finishing to provide a smooth inner surface between the joined tubes.
One type of prior art device for providing such a backing comprises a plurality of firm backing members (typically arcuate copper faced bars) and means for moving the backing members from a release position spaced from the inner surfaces of the tubes, to an engaged position with the backing members forcefully pressed against the inner surfaces of the tubes to be butt-welded. Such devices are suggested in U.S. Pat. Nos. 2,413,103; 2,987,022; 3,369,725; 3,498,518; 3,741,457; and 3,806,020. Typically these devices are quite expensive and do not provide as high a degree of adjustability to accommodate tubes of various inside diameters (e.g., pipe of the same nominal size but of different schedule numbers) or tubes which are out of round. The rigid backing members will not conform to rough inner surfaces so that surface irregularities may space the outer surface of one or more of the backing members from the inner surface of the tubes and allow excessive intrusion of the molten weld material into the joined tubes. Also the backing members occasionally adhere to the weld which interferes with the removal of the device.
It is also well known in the prior art to use a steel ring to provide a backing for fusion butt-welds between the ends of cylindrical tubes. The steel ring is selected to have an outside diameter of about the same size as the inside diameter of the tubes, and a sufficient peripheral width to bridge between the ends of the tubes. The molten weld material adheres to the ring, however, and removal of such a ring, if necessary, requires grinding it out of the pipe, which is expensive and time consuming.
U.S. Pat. No. 3,378,852 teaches using a strip of a flexible, deformable, refractory material comprising inorganic particles bound together with a flexible organic binder as a backing for fusion butt-welds. This refractory material provides an effective and efficient backing, however, applying the refractory material through the use of an adhesive coated strip, as taught in that patent, is not always possible or as convenient as may be desired for applying the refractory material around the inner surfaces of butted tubes.
U.S. Pat. Nos. 2,792,626 and 3,460,736 suggest devices for pressing a strip of backing material into engagement with the inner surfaces of two butted tubes to be butt welded. These devices, however, do not provide the degree of adjustability required for tubes of various sizes. Also they do not provide the proper type of support for the friable refractory material taught in U.S. Pat. No. 3,378,852 while the device is being positioned in butted tubes to be welded, since the device of U.S. Pat. No. 3,460,736 does not maintain an arcuate periphery when not positioned in a tube, and the device of U.S. Pat. No. 2,792,626 maintains the strip of backing material too closely adjacent the inner surface of the tubes to preclude contact therebetween.